The present invention relates to a mechanically operated pressure sensitive switch which is constructed in such a manner that an electric contact and an electric isolation between two opposed electrodes are established when a force is applied to one of the electrodes and when the force is released from the same, respectively. In particular, the present invention is directed, for example, to a flat or sheet shaped switch adapted to be provided in a seat cushion for indicating whether an occupant is on the seat cushion or not by an on/off binary code.
A conventional pressure sensitive sensor for detecting an occupant on a seat includes a magnet generator provided in a seat cushion and a magnet detector which is set to detect the intensity of a magnetic field generated by the magnet generator. When the occupant sits on the seat cushion, the resulting deformation or downward movement of the seat cushion changes the intensity of the magnetic field. As soon as such a change is detected by the magnet receiver, a controller to which the magnet receiver is connected stores this fact for further use.
Another similar conventional pressure sensitive sensor is disclosed in Japanese Patent No. 2535120. This sensor is in the form of a piezoelectric film sheet which is adapted to be provided in a seat cushion. Whenever an occupant sits on and moves away from the seat cushion, a change in force applied thereto varies and such a change is detected as a resistance value change in this sensor.
In addition, Japanese Patent Publication No. Hei2-49029 discloses a pressure sensitive switch. Unlike the foregoing pressure sensitive switches, this pressure sensitive switch is in association with a key board of an electric musical instrument which has a plurality of keys in a row and has a pair of opposed conductive plates between which a semiconductor is interposed. When a force is applied to each key, a resistance value between the conductive plates varies and on the basis of the resulting variation, an application of the force is detected. However, in the first mentioned conventional pressure sensitive switch, the sitting of the occupant on the seat is detected by the lowering movement of the seat per se. For assuring precise detection of such sitting movement by the occupant, a reliable downward movement of the seat has to be established which requires a device which is complex in construction and expensive to manufacture.
As for the second mentioned conventional pressure sensitive switch, the piezoelectric film is an essential component which is very expensive and requires a complex feedback loop electric circuit for oscillating the piezoelectric element continually and a filter for detecting such an oscillation. As a result, the device per se becomes complex in construction which results in a high cost to manufacture.
With respect to the last mentioned conventional pressure sensitive switch, this device is established on the assumption that the force is applied from the finger of a musician to each key. This means that each key is not expected to receive a large force equivalent to the occupant's weight and thus it is very difficult to use the switch for detecting the sitting of the occupant on the seat from the view points of durability, shock-proof, reliability with time and so on. In addition, if copper or silver is employed as the raw material of the conductive plate, the cost of the switch per se becomes very expensive. In brief, the conventional pressure sensitive sensors are not simple in construction, reliable or of low manufacturing cost.